In recent years, a laser apparatus which has a small size but high output power and high beam quality is demanded in a laser machining field and so on. Conventionally, a gas laser has been widely used for a high output power laser, from the viewpoint of the output and the beam quality. However, recently, a laser diode (LD) pumping solid-state laser which uses laser diodes as excitation light sources has spread rapidly.
In order to realize a high output power laser by using the solid-state laser, discharge of heat generated in the laser gain medium is important.
FIG. 1 shows a rod-type solid-state laser system. Pumping light beams 102 are irradiated on the cylindrical side face of a rod-type laser gain medium 101 and an amplified laser beam 103 is outputted from one of the rod ends. In the laser system of this type, a temperature gradient is generated inside the laser gain material rod 101 in a radial direction. This temperature gradient causes degradation of the quality of the laser beam and a laser output drop. Also, it is necessary to consider the durability of laser gain medium to the temperature gradient and to restrain the output power. Moreover, it is difficult to realize the laser gain medium of a large diameter rod from the viewpoint of crystal growth and there is a limit in realization of high output power.
A slab laser system using a plate-type laser gain medium has been devised to reduce the above-mentioned temperature gradient in the rod-type laser gain medium. In this type of laser system, laser light beam propagates in a zigzag manner while being fully reflected in the laser gain medium and is amplified. In this slab laser system, even if the temperature gradient is generated in the laser gain medium, the influence of the temperature gradient on the laser beam can be canceled since the laser beam propagates in the zigzag manner in the temperature gradient.
A method using a disk-type laser gain medium is known in order to solve a heat problem peculiar to the solid-state laser system. FIG. 2 shows a laser system of a transmission type as an example of the disk-type laser system. A pumping light beam 112 is irradiated to a thin disk-type laser gain medium 111 from outside. A laser beam 114 is supplied from one of the faces of the laser gain medium 111 and a laser beam 113 which has been amplified is outputted from the other face. In this laser system, by employing the laser gain medium of thin disk type, the area of a reception face of the pumping light beam can be made large. Moreover, it is possible to uniformly cool the whole disk face, and generation of the temperature gradient of the laser gain medium can be suppressed.
The disk-type laser systems are mainly divided into a transmission type laser system shown in FIG. 2 and a reflection type (an active mirror type) laser system shown in FIG. 3, depending on a method of amplifying the laser beam.
In the transmission-type solid-state laser system, the temperature in the disk rises with irradiation of the pumping beam 112 to the disk-type laser gain medium 111 so that the temperature gradient is generated. In order to eliminate the temperature rising and the temperature gradient, coolant is supplied to the front and back faces of the disk for heat radiation 115. In the transmission-type solid-state laser system, the laser beam which has passed through the coolant is outputted. For this reason, problems such as warp of the outputted laser beam and limitation on operation (for example, stop the supply of the coolant during the laser oscillation) are caused.
On the other hand, in the active mirror type solid-state laser system shown in FIG. 3, one of the faces of a disk-type laser gain medium 121 is covered with a reflection film, a laser beam 123 is inputted from the face opposite to the reflection film and is outputted. By providing a heat discharging mechanism on the whole face covered with the reflection film, the heat accumulated in the laser gain medium can be efficiently discharged as radiation 125. It is possible to output the amplified laser beam without undergoing influence of the heat discharging mechanism. Also, comparing with the transmission-type laser system, since an optical path of the laser beam is folded, the laser beam can be amplified additionally for one return path. Therefore, when disks with the same shape are used, there is an advantage in larger amplification of the laser beam in the reflection type than in the transmission type.
In conjunction with the above description, the following references are known: Japanese Patent Application Publications (JP-A-Heisei 9-312430 and JP-P2006-237170).
In above-mentioned solid-state laser system, a great deal of heat is generated in the laser gain medium with the optical excitation and the temperature of the laser gain medium rises. Therefore, in order to operate the laser system continuously, it is necessary to cool the laser gain medium to discharge the generated heat.
However, since the thermal conductivity of the solid-state laser gain medium is generally low, the large temperature gradient is caused between the hot inside portion and the cooled surface portion. For this reason, phenomena such as inclination of refractive index, mechanical stress, depolarization, and detuning could happen. As a result, it easily causes the degradation of beam quality, reduction of laser output, and the break-down of the solid-state laser medium. This would cause a large problem when extending the solid-state laser to the high output power.
In the large output power laser, a great deal of energy has to be amassed in the laser gain medium. On the other hand, a technique is known in which multi-stage amplification is applied to attain a large gain. In this case, because laser gain media are independent, the light axis of an input laser beam to be amplified is easily shifted due to mechanical vibration of the laser.
In order to obtain a laser beam of high beam quality, it is required that the laser beam is amplified uniformly in a region of the laser gain medium through which the laser beam propagates. In addition, it is required that a temperature is uniform among gain media, since a temperature gradient is generated as the whole laser gain medium so that adverse influence is given on laser oscillation when the temperature of the laser gain medium largely varies due to the heat through absorption of a pumping light beam.